1. Field of the Invention
The present invention relates to a control circuit for a switching transistor that may be used to perform a switching action, for example, in an inverter circuit or a switching electric power supply.
2. Background Information
A conventional control circuit for a switching transistor is known, for example, from Laid-Open Japanese Patent Application No. 60-139018 and corresponding U.S. Pat. No. 4,639,823, both commonly owned by the assignee of the present application.
A prior art control circuit similar to that disclosed in the foregoing Japanese application and corresponding U.S. Patent is shown in FIG. 1, and FIGS. 2A-2D illustrate waveforms useful for explaining the operation of the control circuit in FIG. 1.
In FIGS. 1 and 2A-2D, items which are the same or equivalent to each other are referenced correspondingly. In FIG. 1, the circuit arrangement includes an NPN bipolar transistor 2 for performing the switching action, a reactor (inductor) 5 connected in series with the emitter of transistor 2 to which a current IL (Ic) from the main circuit is applied, and a base driving unit 1, which is well known, for driving the base-emitter circuit of transistor 2. Base driving unit 1 is connected between to the base of the transistor 2 and the end 5a of reactor 5 which is not connected to the emitter of transistor 2. A conventional snubber circuit 3, comprising for example a capacitor, resistor, diode etc., is connected to the collector of transistor 2 and end 5a of reactor 5. A distributed inductance of the snubber circuit and the conductors connecting the transistor to the snubber circuit 3 are represented by coil 4 connected between snubber circuit 3 and the collector of transistor 2. The energy stored in coil 4 while transistor 2 is turned on is applied to transistor 2 as a voltage spike when the transistor is turned off. Snubber circuit 3 reduces that spike voltage to less than a maximum allowable level. A diode arrangement 6, which may include a plurality of diodes connected in series, is connected between end 5a of reactor 5 and the base of transistor 2. Moreover, diode arrangement 6 is connected in a direction such that the electromotive force of reactor 5 which is generated as the current IL decreases when transistor 2 is switched off is applied to the base of the transistor through the diode arrangement 6.
The operation of the circuit of FIG. 1 will be described with reference to FIGS. 2A-2D which show operational waveforms for portions of the circuit. As shown in FIG. 2B, when transistor 2 is on, a positive current IB is supplied from the base driving unit 1 to the base of the transistor 2, and a negative current is supplied to turn the transistor off. The collector current of transistor 2, which is nearly equal to the reactor current IL, starts decreasing after a storage time (tstg) and during a fall time (tf) when the current is supplied in the reverse direction from the emitter to the base of the transistor to switch the transistor 2 off as shown in FIG. 2A.
At this time, because the current IL of reactor starts decreasing as shown in FIG. 2A, an electromotive force EL is induced in reactor 5 with a polarity in which the terminal connected to the emitter of the transistor 2 becomes negative and the other terminal (5a) becomes positive. The electromotive force EL is given by EQU EL=L*dIL/dt
where L is the inductance of reactor 5 and dIL/dt is the rate of change of current IL through inductance 5. Assuming that the forward voltage of the group of diodes 6 is VF and the voltage across the base and emitter of transistor 2 is VBE while the base driving unit 1 supplies a constant current -IB (inverse bias base current), the output current -IB of base driving unit 1 under the condition of EL&lt;VBE+VF will follow the path of (-) terminal reactor 5.fwdarw.emitter.fwdarw.base.fwdarw.(+) terminal, so as to reduce the fall time (tf) of the collector current Ic and cause EL to increase.
Under the condition of EL&gt;VBE+VF, the output current -IB of the base driving unit 1 will follow the path of (-) terminal.fwdarw.group of diodes 6.fwdarw.(+) terminal. At this time, the current from reactor 5 is led to the base of transistor 2 through the group of diodes, 6, acting to prolong the fall time (tf) of the collector current Ic and cause EL to decrease.
Based on the aforementioned operation, the current IL of reactor 5 shows a fall characteristic which satisfies the condition of EL.apprxeq.VBE+VF. Since EL=L* DIL/dt, the fall characteristic (dIL/dt) may be adjusted by changing the forward voltage VF of the group of diodes 6 or the inductance L of the reactor, provided that VBE.apprxeq.constant.
As above described, according to the prior art, it has become possible to maintain the storage time (tstg) at the time of switching transistor off and to adjust the fall time (tf) of the switching transistor. As a result, the rate of change (di/dt) of the current flowing in a snubber circuit for controlling overloading may be minimized, whereby the snubber circuit can be miniaturized. The quantity of the noise generated or reducible because the collector current drop rate (-dlc/dt) of the transistor 2 at the time of switching the transistor off can be minimized.
In the switching circuit shown in FIG. 1, the electromotive force EL of reactor 5 connected in series to the emitter of transistor 2 has the effect of preventing a rapid decrease of emitter current, acting to maintain the transistor 2 on when the transistor is switched off. On the other hand, a voltage is induced in reactor 5 of a polority in which the terminal connected to the emitter of the transistor 2B and the other terminal thereof become positive and negative, respectively, when the transistor is switched from off to on. Since the positive and negative voltages are different in polarity from the base driving signals acting to switch the transistor 2B on and off, respectively, a problem arises in that the rate of increase of the driving (turn on) signal supplied to the base of the transistor is suppressed, and in turn, there is an increase of turn-on time, and turn-on losses of the transistor result. Furthermore, there is a problem that the number of parts in the circuit are increased by adding a reactor and a diode in the form of discrete components.